Pre-transitional Disks: The Missing Link for Planet Formation in Disks


First Author:
Catherine Espaillat
Email: ccespa AT umich.edu
University of Michigan
500 Church Street, 1013 Dennison Building
Ann Arbor, MI 48109-1042
Coauthors:
Calvet, Nuria, University of Michigan
Luhman, Kevin, Pennsylvania State University
Muzerolle, James, University of Arizona
D'Alessio, Paola, Centro de Radioastronomia y Astrofisica

Abstract

In their initial stages of formation planets should interact with the accretion disk surrounding the newborn star, clearing the material around themselves and leaving behind an observational signature in the form of gaps in the primordial disk. Stars with inwardly truncated disks have been detected with Spitzer IRS spectra and are now labeled as "transitional disks." While planet formation can create the inner hole in these disks, other formation mechanisms such as photoevaporation or the magneto-rotational instability can account for this type of clearing as well. Our analysis of IRAC broad-band photometry and IRS spectra has now isolated the earliest stages of planet formation in the disk. This new class of "pre-transitional disks" has an inner optically thick disk separated from an outer optically thick disk by an optically thin gap. In LkCa 15, one of these proposed pre-transitional disks, detailed modeling of the spectral energy distribution demonstrated that although the near-infrared fluxes could be understood in terms of optically thick material at the dust sublimation radius, an alternative model of emission from optically thin dust over a wide range of radii could explain the observations as well. To unveil the true nature of LkCa 15's inner disk we obtained a near-infrared spectrum using SpeX at the IRTF. We report that the excess near-infrared emission above LkCa 15's photosphere is a black-body continuum at the dust destruction temperature, similar to the excess found in full disks. This excess can only be due to an optically thick disk around the star. This is the first confirmation of an inner primordial disk in an object with a separated outer disk. Physical mechanisms that have been presented to explain disk clearing in transitional disks can now be tested with this new class of disk; forming planets emerge as the most likely explanation.
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